Download speeds are terrible and somehow getting worse

My computer has REALLY been struggling to complete downloads/updates lately, and I have no clue as to why.

When I am installing anything, my entire computer essentially freezes up while the download is running, and I can't use any other systems on my computer at the time. My download speeds fluctuate like crazy, peaking at ~350Mbps and staying there for all of 10 seconds, before dropping back down to <100kbps.

I have been sat here for an hour now trying to install a 654Mb patch for a game. An hour.

Anybody know anything?